Method for separating ammonia from solutions containing caprolactam and ammonia

ABSTRACT

A process for distillative removal of ammonia from solutions (I) which include a lactam and ammonia comprises effecting said removal in a distillation apparatus (a) at an absolute pressure of less than 10 bar.

DESCRIPTION

The present invention relates to a process for distillative removal ofammonia from solutions (I) which include a lactam and ammonia, whichcomprises effecting said removal in a distillation apparatus (a) at anabsolute pressure of less than 10 bar.

Solutions including a lactam and ammonia are produced in numerouschemical processes, for example in the production of cyclic lactams byreaction of omega-aminocarboxylic acid derivatives with water.

Workup of these solutions is problematic, since lactams, especiallycaprolactam, customarily have to meet extremely high purityrequirements.

WO 95/14665 and WO 95/14664 describe reacting 6-aminocapronitrile(“ACN”) in the liquid phase with water in the presence of heterogeneouscatalysts and of a solvent to form a solution including caprolactam andammonia. Workup of this solution is not described.

The distillative removal of ammonia from ammonia-containing solutions isgenerally carried out at high pressure, customarily more than 14 barabsolute, to obtain a suitable temperature for the condensation ofammonia at the top of the distillation apparatus.

It is an object of the present invention to provide a technically simpleand economical process for removing ammonia from solutions including alactam and ammonia.

We have found that this object is achieved by the process defined at thebeginning.

According to the invention solution (I) includes a lactam and ammonia.

In principle there are no known limitations with regard to the lactam.Preference is given to lactams of C₄-C₂₀-omega-carboxylic acids, such as4-aminobutanelactam, 5-aminopentanelactam, 6-aminohexanelactam(“caprolactam”), 7-aminoheptanelactam or 8-aminooctanelactam,particularly preferably caprolactam. These lactams may be substituted,for example by C1-C4-alkyl groups, halogens, such as fluorine, chlorineor bromine, C1-C4-alkoxy groups or C1-C4-carboxyl groups, but preferablythe lactams are not substituted.

Mixtures of such lactams may also be used.

Such lactams are known per se.

Such lactams may be prepared by reacting the corresponding aminonitrileswith water, for example in the case of caprolactam by reaction with6-aminocapronitrile, as described for example in EP-A-0 659 741, WO95/14664, WO 95/14665, WO 96/22874, WO 96/22974, WO 97/23454, WO99/28296 or WO 99/47500.

The reactor exit stream of such a reaction contains at least one mole ofammonia per mole of lactam. The reactor exit stream may include furthercomponents, such as unconverted aminonitrile, excess water, water usedto quench the reactor exit stream in the case of a reaction in the gasphase, or, if appropriate, organic solvents.

The process of the invention may utilize as solution (I) any mixture ofa lactam and ammonia, advantageously one of the reactor exit streamsmentioned, or mixtures of such systems.

According to the invention ammonia is distilled out of solution (I) at apressure of less than 10 bar absolute, preferably less than 8 barabsolute. Advantageously the pressure used is above the atmospheric,ambient pressure.

Distillation pressure and distillation temperature should preferably beselected so that an essentially ammonia stream can be taken off overheadin vaporous form.

The distillation apparatus (a) may be any customary single-stage ormulti-stage distillation apparatus, as described for example in:Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, JohnWiley & Sons, New York, 1979, pages 870-881, such as evaporationchambers or rectification columns, for example sieve plate columns,bubblecap plate columns, orderedly packed columns or randomly packedcolumns.

Single stage distillation chambers, pure stripping column orrectification columns with stripping and enriching sections can be usedhere.

In a preferred embodiment the vaporous essentially ammonia streamobtained at the top of the distillation apparatus (a) may be passedthrough a condenser (ak). In this condenser compounds having a higherboiling point than ammonia may advantageously be partly or completelycondensed to obtain a condensate (III) and an essentially ammoniagaseous phase.

Condensate (III) may advantageously be partly or completely recycledinto the distillation apparatus (a), especially when the distillationapparatus (a) is a column, particularly preferably when this columnincludes a rectifying section.

Ammonia may be partially or completely condensed out of the essentiallyammonia stream from apparatus (a) or condenser (ak) by absorption in aliquid solvent (II). The stream may for this purpose be compressedbeforehand.

The absorber (b) may be any customary single-stage or multi-stagedistillation apparatus, as described for example in: Kirk-Othmer,Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, John Wiley & Sons,New York, 1979, pages 870-881, such as bubble columns or absorptioncolumns, for example sieve plate columns, bubblecap plate columns,orderedly packed columns or randomly packed columns.

It may be advantageous to carry out cooling between the separatingstages of the absorption column (b).

In a preferred embodiment column (a) may be equipped with a rectifyingsection operated with a solvent (IV) as reflux. Preferably the solvent(IV) should be essentially ammonia free. Advantageously a portion of thesolvent (II) loaded with ammonia in the absorption stage may be used asreflux.

Advantageously solvent (II), solvent (IV) or both solvents are water or,when the lactam is prepared from the corresponding aminonitrile, thesolvent used in this reaction.

In a preferred embodiment ammonia is removed from the column (b) loadedsolvent (II) by distillation or rectification in a distillationapparatus (c).

The distillation apparatus (c) may be any customary single-stage ormulti-stage distillation apparatus, as described for example in:Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Ed., Vol. 7, JohnWiley & Sons, New York, 1979, pages 870-881, such as evaporationchambers or rectification columns, for example sieve plate columns,bubblecap plate columns, orderedly packed columns or randomly packedcolumns.

Single-stage distillation chambers, pure stripping colums orrectification columns with stripping and enriching sections can be usedhere.

Advantageously said removal of ammonia in said apparatus (c) is effectedunder a higher pressure than the pressure in said column (a), preferablymore than 10 bar absolute.

A preferred embodiment contemplates the compression, especially to apressure of more than 12 bar absolute, and partial or completecondensation of the vaporous essentially ammonia stream from column (a)or condenser (ak).

A portion of the condensed stream may advantageously be recycled tocolumn (a) as reflux. Flash evaporation with corresponding cooling ofthe reflux stream is contemplated.

Useful pressures for a compression stage are especially those pressuresunder which ammonia is liquid at from 0 to 50° C. Such temperatures maybe provided in a technically simple manner by cooling with river wateror air for example.

Compression may be effected using conventional apparatuses, such asturbo, piston, membrane or preferably liquid ring compressors.

In a preferred variant the compressing may be effected using a liquidring compressor, in which case this liquid ring compressor may becharged with solvent (II), especially with ammonia-loaded solvent (II)from the absorption stage.

The overflowing liquid from the liquid ring compressor may be partiallyor completely recycled to column (a) as reflux.

The process of the invention has the advantage in relation to thedistillative removal of ammonia from solutions that include lactam,especially caprolactam, and ammonia and that come from the production oflactam by reaction of aminonitrile with water, of preventing theformation of solids in the distillation column which hinders continuousoperation of the workup.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 4 illustrate the various embodiments of the process addressedin the foregoing.

COMPARATIVE EXAMPLE

100 kg/h of ACN were converted to caprolactam in 1000 kg/h of ethanolover an oxidic catalyst in the liquid phase. The reactor exit streamcontained 12% by weight of ammonia and was directed for distillationinto a sieve plate column containing 20 sieve plates. The top of thecolumn was fitted with a condenser. The column was operated at apressure of 15 bar absolute. The liquid ammonia obtained as condensatefrom the condenser was partially recycled as reflux to the top of thecolumn at a rate of 200 kg/h. The ammonia content of the stream takenoff at the base of the column was less than 1000 ppm. 30 h after thecolumn had been started up the sieve plates became clogged up, which wasdiscernible from the fact that the pressure difference between base andtop increased dramatically for the same space velocity through thecolumn. The column had to be switched off and purged with water.

Inventive Example

The liquid reactor exit stream from the comparative example was directedinto the same column, but this column was operated at a pressure of 5bar absolute. The vapor stream from the top of the column was passedthrough the condenser without, however, condensing out all the ammonia.The remaining vaporous ammonia was taken off and the liquid condensaterecycled to the column as reflux. Column operation was stable forseveral weeks.

1. A process for distillative removal of ammonia from a solution (I)which comprises a lactam and ammonia, which process comprises effectingsaid removal in a distillation apparatus (a) at an absolute pressure ofless than 10 bar, to obtain a vaporous stream comprising ammonia,introducing said vaporous stream into a condenser (ak) in whichcomponents having a boiling point higher than that of ammonia are partlyor completely condensed to form a condensate (III) of components havinga higher boiling point than ammonia, and a vaporous stream consistingessentially of ammonia, and recycling said condensate (III) from saidcondenser (ak) into the distillation apparatus (a).
 2. A process asclaimed in claim 1, wherein said removal is effected at an absolutepressure of less than 8 bar.
 3. A process as claimed in claim 1, whereinan essentially ammonia stream is taken off overhead in vaporous form. 4.A process as claimed in claim 1, wherein said vaporous ammonia streamobtained from said condenser (ak) is subsequently condensed by absorbingammonia in a liquid solvent (II).
 5. A process as claimed in claim 4,wherein said solvent (II) is water.
 6. A process as claimed in claim 4,wherein ammonia is removed from the solvent (II) containing absorbedammonia by distillation or rectification in a distillation apparatus(c).
 7. A process as claimed in claim 4, wherein ammonia is removed fromthe solvent (II) containing absorbed ammonia in a single evaporationstage.
 8. A process as claimed in claim 4, wherein said solution (I) isa solution obtained in the production of a lactam from an aminonitrile.9. A process as claimed in claim 8, wherein said solvent (II) is thesame solvent as used in the production of the lactam from thecorresponding aminonitrile.
 10. A process as claimed in claim 4, whereinsaid absorption takes place in an absorber (b).
 11. A process as claimedin claim 10, wherein said vaporous ammonia stream and the liquid solvent(II) are cooled in said absorber (b).
 12. A process as claimed in claim10, wherein said distillation apparatus (a) includes a rectifyingsection which is operated with a reflux medium, said reflux medium beinga portion of the solvent (II) containing absorbed ammonia which isformed in said absorber (b).
 13. A process as claimed in claim 1,wherein said distillation apparatus (a) includes a rectifying sectionwhich is operated with a solvent (IV) as a reflux medium.
 14. A processas claimed in claim 13, wherein said solvent (IV) is essentially ammoniafree.
 15. A process as claimed in claim 13, wherein said solvent (IV) iswater.
 16. A process as claimed in claim 13, wherein said solution (I)is a solution obtained in the production of a lactam from anaminonitrile.
 17. A process as claimed in claim 16, wherein said solvent(IV) is the same solvent as used in the production of the lactam fromthe aminonitrile.
 18. A process as claimed in claim 1, wherein saidvaporous stream obtained from said condenser (ak) is compressed in acompressor (comp) and is partly or completely condensed.
 19. A processas claimed in claim 18, wherein said distillation apparatus (a) includesa rectifying section which is operated with a reflux medium, said refluxmedium being a portion of said condensed stream which is recycled intosaid distillation apparatus (a) for a flash evaporation withcorresponding cooling of the reflux medium.
 20. A process as claimed inclaim 18, wherein said compressor (camp) is a liquid ring compressor.21. A process as claimed in claim 20, wherein said liquid ringcompressor is charged with a solvent (II).
 22. A process as claimed inclaim 1, wherein said lactam is caprolactam.
 23. A process fordistillative removal of ammonia from a solution comprising ammonia,which process comprises effecting said removal in a distillationapparatus (a) to obtain a vaporous stream which comprises ammonia,introducing said vaporous stream in a condenser (ak) to form acondensate of compounds having a boiling point which is higher than theboiling point of ammonia and a vaporous stream consisting essentially ofammonia, and subsequently absorbing the ammonia of said vaporous ammoniastream obtained from said condenser (ak) in a liquid solvent (II).